NoSimpleLetBody should report destructuring for named patterns where the type variable is not a phantom type

[jfmengels]

At the moment, the rule doesn't report the destructuring of single variant constructors when we don't know whether the type has any phantom types (as that could be the purpose of the destructuring).

This correctly doesn't get reported.

type A b = A String
a = let (A b) = xyz
    in A b

but the heuristic we use is to only check whether the type has type variables. We are not checking whether the type variable is actually a phantom one or not.

module A exposing (..)
type A b = A b
-- should be simplified
a = let (A b) = xyz
    in A b

There may some edge cases to figure out where this might be change things? For instance, what if the type is a phantom type by transition (used as a type variable to another type where it is a phantom type)? To be explored.